House electric



.R. T. PIERCE.

CONTROL SYSTEM FOR ELECTRIC FURNACES. APPLICATION FILED DEC. 14. 1918.

1,367,089? Patented Feb. 1 1921.

WITNESSES): lNVENTOR Raymond P I ATTORNEY UNITED STATES PATENT orrice.

HOUSE ELECTRIC & MANUFACTURING COMPANY, A CORPORATION OF PENNSYIR VANIA.

Specification of Letters Patent.

CONTROL SYSTEM ron ELEc'rnic rURN AcEs.

Patented Feb. 1, 1921.

Applicationfiled December 14, 1918. .Serial No. 266,688.

To all whom it mayjcon-cem;

Be it known that 1 RAYMON T.'PIERCE, a citizenof the Unitedstates, and aresident of Willt'insburg'in the county of Allegheny and State ofPennsylvania, have invented a new and useful Improvement in ControlSystems for Electric Furnaces; of which the following is aspecification.

My invention relates to control systems for electric furnaces and it hasparticular relation to the control of those furnaces in which a negativetemperature coefficient exists during the operation thereof.

In furnaces of the character designated, it is necessary to maintain asubstantially constant temperature after the initial setting, thetemperature depending upon the character of work being done, the time offiring, etc. It is a matter of more or less difiiculty, in suchfurnaces, to maintain the desired temperature and, at the same time,have available a simple and compact means for preventing an excessiveoverload upon the furnace and the 'subsequent burning-out thereof. It isobvious, of course, that some danger will always exist in furnaces ofthe type described because of the negative temperature coeflicientobtaining therein. That is. as the furnace becomes hotter, theresistance thereof tends to decrease, and the current passing throughthe furnace, therefore, increases.

I, therefore, arrange a control system in which I employ a minimum.number of switching arrangements to accomplish the desired control, and,at the same time, I so associate these switches with a safetyinterlocking member that the burning out of the furnaces, by reason ofan excessive overload being imposed thereon, w1ll be prevented.

Another object of my invention is to provide'a positivecurrent-controlled element which shall operate to limit the currentpass-v ing through the furnace.

For a better understanding of my invention, reference may be had to theaccompanying drawing, the single figure of whlch illustrates a controlsystem constructed -]I1 accordance with my invention. A furnace 1, isshown as supplied with power from a secondary member 2 of a transformer3, the primary member of which 1s connected to v any suitablesource ofenergy.

t0 the furnace may be Varied in The secondary winding 2 of the transformer 3 is provided with spaced taps 4, 5 and 6, these taps being soassociated with the winding that the number of effective turns thereofwhich are used to feed power accordance wlth the control desired.

The control elements are embodied in a graphic-ammeter arrangement whichcomprises electro-dynamometer elements 7 and S controlling the rotationof a motor 9, the

motor 9 being provided with a worm-w threaded shaft 10.

The manner in which a recording ammevter, of the type designated,operates is set forth in numerous. U. S patents, the best showingprobably being that of U. S. Patent N 0. 1,010,160, issued to theWestinghouse Electric & Manufacturing Company, November 28th, 1911, onan application filed by Paul MacGahan. In that patent is set forth themanner in which the control 'cir-- cuits are :associated with a Kelvinbalance, the movable member of which is adapted to make contact betweentwo terminal points in such way that a motor is constrained to run inthe one or the other direction. However, the structure shown in thatpatent contemplates the use of a continuously operating motor, and meansare provided whereby the recording mechanism is engaged-by, anddisengaged from, the continuously rotating worm shaft upon which themotor is mounted. In the present applica- I tion of the Kelvin balance,recording-ammoter structure, I contemplate the use of a motor which soperated in the one or the other direction by the, coaction of themovable arm of the aforedescribed balance system,

and, in place of the recording pen, I'use an engaging element whichactuates switches to control the amount of voltage supplied to thefurnace, all of which is hereinafter described in detail. The members 7and.8 are energized, in accordance with the current passing through thefurnace, by means of a current transformer'll'disposed in one of theleads passing from the secondary winding of the transformer 2 to thefurnace.

A movable member or carriage 12 is mounted upon the worm-threaded shaft10 and is adapted to move along the same. The carriage 12 is providedwith a .lug 13 which is adapted to engage .and operate certain 16, 17and 18, hereinbefore described, are

operated to vary the desired number of effective controls existing uponthe transformer secondary 2.

It will be noted that the three-point switches 20 and 21 are provided,respectivel with. biasing springs 22 and 23, thus making their actionmore positive and satisfactory. I the switching levers of the switches20 and 21 through engaging pin members 25, 26, 27 and 28 in a mannerhereinafter described.

Having described a control system con- 'structed in accordance with myinvention,

the. operation thereof is as follows:

Assuming thatthe main-line switch to the furnace 1 has been closed and,moreover, that the control circuit has been energized, a circuit will becompleted through the energizing coil of the switch 16. The closure ofthe switch 16 causes a maximum number of effective turns to be active insupplying power to thefurnace, and the current passing through thefurnace Wlll, therefore, be oflarge value. As the furnace heats up,however, the resistance thereof decreases, since it'possesses a negativetemperature coefficient and th'erefore, the current passing through theeurnace willincrease more than is desired. This increase of current, byits action upon the current transformer 11,

will cause the motor 9 to rotate and the member 12 to travel along theworm-threadedv shaft-1O. The lug 13 on the carriage 12' will then engagethe switching lever of the switch 20 and will .snap the same fromthe-full-hne position shown in the drawing to that indicated by dottedlines. This action completes a circuit through the switch 17. to placealesser number of effective turns of the secondary winding 2 of thetransformer 3 into operation, thus causing the current to reassume anormal value. However, it should be noted that, if the current nowdrops.

enough to cause the carriage 12 to again make contact with the upperterminal 14, no change will occur I nov longer in such position thattheswitch 16 may be actuated.- Moreover, it should.

be noted'in connection with the'sub sequent description of theother'switchescontrolhng A member 2-1 is adapted to operate since theswitch 20 is;v

ne .of said windings the effective turns of the transformer, that thissafety relation always exists.

Assuming now that the current continues to increase by reason of theincreasing temperature in the furnace, the carriage 12 will move stillfarther to the right, and the lug 13 will engage the switching lever 21and move it from its full-line position to that shown by dotted lines. Acircuit will now be completed through the coil operating the switch 18.and ,the least number of effective turns of the secondary 2 ofthetransformer 3 will be connected to the furnace.

At this point, the temperature of, and the current passing through, thefurnace should become so stabilized that no further increase oftemperature should occur. However, if

a further undesirable increase does occur,

and, if on account of the increasing danger of an abnormal current, itis desired to dis connect the furnace and allow it to cool slightly, thecarriage 12, in itscontinued movement to the right, will cause the lug13 to engage the member 24 and, since the switching members to which themember 24 is attached are provided with snap springs, a slight movementof the member 24 will cause the switches 20 and 21 to reassume thepositions noted at the beginning of the description of the operation ofthe system.

Since all the switches 16, 17 and 18 are now open. no power will be fedto the furnace, and it will gradually cool. Moreover, since no currentis now flowing into the furnace, the coil. 11 will be deenergized, andthe motor will rotate in such direction as to cause the carriage 15 totravel toward the left. As soon as the carriage 15 makes contact withthe contact member 14, the switch 16 will be closed and the furnace willagain be supplied with power.

The above set of conditions will result in the furnace again graduallyheating up, but, in the interval of time elapsing between the engagementof the carriage 12 with the member 24 and its subsequent backward travelto the point where it will engage the contact member 14, the furnaceW111 have had an opportunity to so far cool off, that the resistancethereof will be again decreased and the current will, therefore,gradually become stabilized.

While I have shown but one embodiment of my invention in whichmotor-controlled contact members are used, it is obvious that manymodifications therein. may occur to those skilled in the art, and Idesire, therefore, that only such limitations be placed on or by theappended claims. I

I claim as my invention: 1.' In a regulating s'ystem furnace, thecombination with a transformer having a primary and a secondary winding,

my invention as are imposed by the priorart for. an electric beinnrwifilelv wit a 1 6i;-

plurality of taps, and'switches'for connecting said taps to the furnace,of motor-operated means for controlling said switch-es to maintain asubstantially-constant3*current flow through the furnace andjfor openingthe furnace circuit under certain'conditions, 2. In a regulating systemfor an electric furnace, the combination with a transformer having aplurality of taps extending from it, and switches for connecting saidtaps tov the furnace, of motor-operated means for operating saidswitches in accordance with the electrical conditions of said furnace,and

means for opening the circuit of the furnace in case a current abovenormal value passes through the furnace: for an extended period nace anda transformer having an adjust able secondary winding for supplyingpower thereto, of means comprising a motor for controlling the powersupplied to said furnace by said transformer secondary winding, saidmotor being operated in a direction cor-f responding to the amount ofcurrent passing- .through' said furnace.

, 5.'The combination with an electric 'furnace and a transformer havingan ad ust able secondary winding for supplying power thereto, of a motoroperated in a direction corresponding to the current passing throughsaid furnace, and contact-making means associated with said motor forcontrolling the number of effective turns in said secondary winding. v

6. The combination with 'an electric furnace and a transformer having anadjustable secondary winding for supplying ower thereto, of a motoroperated. in a-direction corresponding to the current passing throughsaid furnace, the shaft of said motor being provided with a worm thread,and

a movable engaging member fadaptedto move along said worm shaftand-operating to change the effective turnsof said transformer secondarywinding.

7. The combination with an electrlc far nace and a transformer having anadjustable secondary winding for supplying power thereto, of amotonh'aving a wormthreaded shaft, an engaging j contact member adaptedto move. along said shaft, and switches controlling the effective turnsof said transformersecondary winding, said switches being operated bysaid movable contactmember.

'8, The combination with an electric furnace and a transformer having anadjustable secondary winding for supplying power thereto, of a motorpamovable contact member controlledby the rotation of saidmotor, switchesactuated thereby and controlling the. eife'ctive turns of saidtransformer secondary winding; and a member operated by said contactmeir'nberv for controlling said switches whereby, upon the occurrenceof. an abnormal overload on said transformer secondary winding, no poweris supplied to said furnace.

9. The combination with an electric furnace and a transformer having anadjustable secondary winding for supplying power thereto, ofmotor-operated means for gradually decreasing the number of effectiveturns of said transformer secondary wmdmg as the current in said furnacetends to rise, said 'means being-. provided with a safety element to'renderLsaud secondary winding completely neffective when an abnormalload is imposed thereon:

1o.- The cOmbinatiOnwithanelectric furnace and a transformerhaving'anadjust- Y able secondary winding"; for supplying power thereto, ofmotor-operated means for gradually decreasing the numberjof effectiveturnsof said transformer secondary ,Wll'idmg as the current in saldfurnace tends to rise, said means. being provided .with a safety elementto render said secondary wlnding completely ineffective when-an ab-inormal load is imposed thereon and-beingautomatically operative torestore correct temperatureconditions upon the cessation of saidabnormal load.

In testimony whereof, I have hereunto subscribed my name this 2nd dayof.Dec.,

n-AYMoNn T. PIERCE;

